Acids, Bases and Salts — Class 10 Science
Science · Chapter 2
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Chapter summary
Key ideas & terms
- pH scale
- A 0–14 scale measuring acidity (low) or basicity (high); 7 is neutral.
- Neutralisation
- Reaction of an acid with a base to give salt and water.
- Indicator
- A substance that changes colour to show acidic or basic nature.
- Water of crystallisation
- Fixed water molecules present in a crystalline salt.
Extra questions & answers
What happens to the pH when an acid is diluted with water?
The concentration of H⁺ ions per unit volume decreases, so the pH rises slightly towards 7.
Why does dry HCl gas not turn blue litmus red?
Because it does not release H⁺ ions in the absence of water; acidic behaviour needs water to ionise.
How do acids and bases behave in water?
Acids produce hydrogen ions (H⁺) in water while bases produce hydroxide ions (OH⁻); water-soluble bases are called alkalis.
What are indicators? Name a few.
Substances that show whether a solution is acidic or basic by a change of colour or smell, such as litmus, phenolphthalein, methyl orange and olfactory indicators like onion.
What is the pH scale, and why is it important?
A scale from 0 to 14 measuring the acidity or basicity of a solution (7 is neutral); it matters in digestion, soil for crops, tooth decay and survival of organisms.
How do acids react with metals and with metal carbonates?
Acids react with active metals to give a salt and hydrogen gas, and with metal carbonates/bicarbonates to give a salt, water and carbon dioxide.
What is neutralisation? Give the general reaction.
The reaction between an acid and a base to form salt and water: acid + base → salt + water; the acidic and basic natures cancel out.
How are baking soda and washing soda made and used?
Baking soda (sodium hydrogencarbonate) is made from brine, ammonia and CO₂, and used in cooking and antacids; washing soda (sodium carbonate) is got from baking soda and used for cleaning and softening water.
What is bleaching powder, and how is it prepared?
Calcium oxychloride, prepared by passing chlorine over dry slaked lime; it is used for bleaching and disinfecting water.
What is water of crystallisation? Give an example.
A fixed number of water molecules present in one formula unit of a salt, e.g. copper sulphate crystals (CuSO₄·5H₂O), which turn white on heating.
QUESTION
1. You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?
Dip separate strips of red litmus paper into the three solutions. The solution that turns red litmus blue is the basic solution. Now use the blue litmus strip produced in this test to test the other two solutions. The solution that turns the blue litmus red is the acidic solution, while the solution that causes no colour change is distilled water.
QUESTIONS
1. Why should curd and sour substances not be kept in brass and copper vessels?
Curd and other sour substances contain acids. These acids can react with brass or copper to form harmful metal salts, which may contaminate the food. Therefore, such substances should not be kept in brass or copper vessels.
2. Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Hydrogen gas is usually liberated when an acid reacts with a metal. For example, zinc reacts with dilute sulphuric acid as follows:
3. Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.
The gas that extinguishes a burning candle is carbon dioxide. Since calcium chloride is formed, compound A may be calcium carbonate. The balanced equation is:
QUESTIONS
1. Why do
In water, compounds such as
2. Why does an aqueous solution of an acid conduct electricity?
An acid ionises in water and produces mobile ions such as
3. Why does dry
Dry
4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Diluting a concentrated acid releases a large amount of heat. When acid is added slowly to a larger quantity of water with continuous stirring, the heat is absorbed and dispersed safely. If water is added to concentrated acid, sudden local heating may cause the mixture to boil and splash, resulting in burns or breakage of the container.
5. How is the concentration of hydronium ions
On dilution, the volume of the solution increases while the number of hydronium ions does not increase proportionately. Therefore, the concentration of
6. How is the concentration of hydroxide ions
Dissolving more base in the sodium hydroxide solution produces additional
QUESTIONS
1. You have two solutions, A and B. The pH of solution A is
Solution A has the greater hydrogen-ion concentration because a lower pH corresponds to a higher concentration of
2. What effect does the concentration of
An increase in the concentration of
3. Do basic solutions also have
Yes, basic aqueous solutions also contain some
4. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?
A farmer would add quick lime, slaked lime or chalk when the soil is too acidic and has a low pH. These basic substances neutralise excess acidity and raise the soil pH to a range more suitable for plant growth.
QUESTIONS
1. What is the common name of the compound
The common name of
2. Name the substance which on treatment with chlorine yields bleaching powder.
Dry slaked lime,
3. Name the sodium compound which is used for softening hard water.
Washing soda, or sodium carbonate decahydrate,
4. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
On heating, sodium hydrogencarbonate decomposes to form sodium carbonate, water and carbon dioxide.
5. Write an equation to show the reaction between Plaster of Paris and water.
Plaster of Paris reacts with water and changes into hard, solid gypsum:
EXERCISES
1. A solution turns red litmus blue, its pH is likely to be
(a)
(d)
2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
(a)
(b)
3.
(a)
(d)
4. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
(c) Antacid. Antacids are mild bases that neutralise excess acid in the stomach and provide relief from indigestion.
5. Write word equations and then balanced equations for the reaction taking place when –
(a) dilute sulphuric acid reacts with zinc granules.
Word equation: Zinc + sulphuric acid → zinc sulphate + hydrogen. Balanced equation:
(b) dilute hydrochloric acid reacts with magnesium ribbon.
Word equation: Magnesium + hydrochloric acid → magnesium chloride + hydrogen. Balanced equation:
(c) dilute sulphuric acid reacts with aluminium powder.
Word equation: Aluminium + sulphuric acid → aluminium sulphate + hydrogen. Balanced equation:
(d) dilute hydrochloric acid reacts with iron filings.
Word equation: Iron + hydrochloric acid → iron(II) chloride + hydrogen. Balanced equation:
6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Take separate aqueous solutions of dilute hydrochloric acid, glucose and alcohol in three beakers. Fix two nails in a cork and connect them to a battery, bulb and switch to form a circuit. Place the nails in each solution one at a time without allowing them to touch. The bulb glows in the hydrochloric acid solution because the acid produces mobile ions in water. It does not glow in the glucose or alcohol solutions because these substances do not ionise to produce
7. Why does distilled water not conduct electricity, whereas rain water does?
Distilled water contains almost no dissolved ions, so it does not conduct electricity effectively. Rain water dissolves gases, dust and salts from the atmosphere, producing ions in the water. These ions carry electric current, so rain water conducts electricity.
8. Why does dry
Acidic behaviour requires the formation of hydrogen or hydronium ions. Dry
9. Five solutions A,B,C,D and E when tested with universal indicator showed pH as
(a) neutral?
Solution D is neutral because its pH is
(b) strongly alkaline?
Solution C is strongly alkaline because its pH is
(c) strongly acidic?
Solution B is strongly acidic because its pH is
(d) weakly acidic?
Solution A is weakly acidic because its pH is
(e) weakly alkaline?
Solution E is weakly alkaline because its pH is
Arrange the pH in increasing order of hydrogen-ion concentration.
Hydrogen-ion concentration decreases as pH increases. Therefore, the pH values in increasing order of hydrogen-ion concentration are:
10. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid
Fizzing will occur more vigorously in test tube A containing hydrochloric acid.
11. Fresh milk has a pH of
The pH will fall below
12. A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from
Baking soda is mildly basic. It makes the milk slightly alkaline so that the lactic acid formed by bacteria must first neutralise the added base before the milk becomes sufficiently acidic to turn into curd. This delays souring.
(b) Why does this milk take a long time to set as curd?
The lactic acid produced in the milk initially neutralises the baking soda. Only after enough acid has formed to neutralise the base can the pH fall to the level needed for curd formation. Therefore, the milk takes longer to set as curd.
13. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Plaster of Paris reacts readily with moisture and changes into gypsum, forming a hard solid mass:
14. What is a neutralisation reaction? Give two examples.
A neutralisation reaction is a reaction in which an acid and a base react to form salt and water. Examples are:
15. Give two important uses of washing soda and baking soda.
Two uses of washing soda are removing permanent hardness of water and serving as a cleaning agent or raw material in glass, soap and paper industries. Two uses of baking soda are as a component of baking powder for making bread and cakes soft and spongy, and as an antacid for neutralising excess stomach acid.
Group Activity
(I) Prepare your own indicator
Crush beetroot in a mortar.
Add sufficient water to obtain the extract.
Filter the extract by the procedure learnt by you in earlier classes.
Collect the filtrate to test the substances you may have tasted earlier.
Arrange four test tubes in a test tube stand and label them as A,B,C and D. Pour
Put
You can prepare indicators by using other natural materials like extracts of red cabbage leaves, coloured petals of some flowers such as Petunia, Hydrangea and Geranium.
Beetroot extract acts as a natural acid-base indicator. A suitable observation table is: Test tube A—lemon juice solution: acidic; the beetroot colour remains reddish or pink. Test tube B—soda-water: mildly acidic; the extract remains reddish or pink with a slight change in shade. Test tube C—vinegar: acidic; the extract remains reddish or pink. Test tube D—baking soda solution: basic; the colour changes towards a darker purplish or yellowish-brown shade depending on the concentration of the extract. The exact shades may vary, but the acidic samples show a colour different from the basic baking-soda solution.
(II) Preparing a soda-acid fire extinguisher
The reaction of acids with metal hydrogencarbonates is used in the fire extinguishers which produce carbon dioxide.
Take
Suspend an ignition tube containing dilute sulphuric acid in the wash-bottle (Fig.
Close the mouth of the wash-bottle.
Tilt the wash-bottle so that the acid from the ignition tube mixes with the sodium hydrogencarbonate solution below.
You will notice discharge coming out of the nozzle.
Direct this discharge on a burning candle. What happens?
The candle flame is extinguished. Dilute sulphuric acid reacts with sodium hydrogencarbonate and releases carbon dioxide: